Electrochemical cells using metal as a fuel are known. Electrochemical cells using an electrolyte, a solution of solvent molecules and solute ions, as an ionically conductive medium are also known. Electrolytes maintain ionic conductivity as solvent molecules solvate solute ions due to thermodynamic interactions between those species. Electrochemical cells using metal as a fuel may be “primary” (i.e. non-rechargeable) or “secondary” (i.e. rechargeable) cells depending on desired operating characteristics and chemistries. In electrochemical cells using metal as the fuel, metal fuel is oxidized during discharge at a fuel electrode functioning as an anode. The oxidized metal fuel ions may remain in the electrolyte solution in reducible form (either as solvated ions, or combined with other ions, such as in a molecule or complex).
During charging of secondary electrochemical cells, reducible metal fuel ions are reduced to metal fuel at the interface between the electrolyte and the fuel electrode, which is now functioning as a cathode; the metal fuel thus plates the fuel electrode by this process, known as electrodeposition.
A significant problem for electrochemical cells comprising a metal fuel is the tendency for corrosion or self-discharge during idle modes (e.g. storage). This most often translates to a loss in usable capacity. In more extreme cases, self-discharge may result in outgassing and excess pressures may rupture cell seals, ultimately causing cell failure. For example, in alkaline batteries comprising zinc metal fuel, the native oxide layer is insufficient to stop the corrosion process which often results in cell performance losses.
For secondary batteries, a significant problem that arises upon charge-discharge cycling is the formation of filaments or dendrites. These formations are often nonuniform, disperse deposits which may be due to mossy or dendritic growth, and/or due to the growth of filaments, nodules, etc. Often this type of metal deposition may cause an undesirable short-circuit between the electrodes resulting in cell failure. Ideally, the electrodeposited metal accumulates as a smooth layer over the entire fuel electrode surface, thereby preserving the electrode surface morphology from one charge-discharge cycle to the next.
Another problem associated with conventional aqueous electrolyte batteries, is water electrolysis during charging. During charge, a current is passed through the battery to reduce the oxidized fuel at the fuel electrode. Some of the current, however, electrolyzes the water resulting in hydrogen evolution (i.e. reduction) at the fuel electrode and oxygen evolution (i.e. oxidation) at the oxidant electrode as represented in aqueous alkali by the following equations:2H2O(l)+2e−→H2(g)+2OH−(aq) and  (1)2OH−(aq)→½O2(g)+H2O(l)+2e−.  (2)In this manner, aqueous electrolyte is lost from the battery. Additionally, the electrons that are consumed in reducing hydrogen are not available to reduce the fuel at the fuel electrode. Therefore, the parasitic electrolysis of the aqueous electrolyte reduces the round trip efficiency of the secondary (i.e. rechargeable) battery.
To mediate these problems, the electrolyte solution may comprise an additive. Electrochemical cells using an additive are known. Examples of such devices are shown, for example, in U.S. Pat. Nos. 3,945,849; 4,479,856; 6,027,827; and 6,395,422; 6,927,000; 7,169,504; and 7,563,537 which are incorporated herein in their entirety. Additives for different electrochemical systems may include surfactants, metal ions, organic compounds and combinations thereof. Benefits of additive use in an electrochemical cell may, for instance, improve the electrochemical reactions by various means, for example, forming an ionically conductive layer on an electrode, adsorbing on the metal surface and preventing corrosion, decreasing wettability issues of electrodes or acting as a chelating agent. Yet, the additive may, in result, impede the function or efficiency of the electrochemical cell. For example, an electrolyte in a regenerative cell that promotes quick electroplating may concurrently promote less dense electroplating of the metal fuel on an electrode. As another example, strong adsorption of an additive may require higher overpotentials during charging of a secondary battery, thus decreasing efficiency.